For years, Facebook was considered the happiest place to work in Silicon Valley. It ranked Number 1 among tech workplaces on Glassdoor. No one who got a job there ever seemed to quit.

Then 2018 happened. There was a long list of scandals. #DeleteFacebook became a thing. Mark Zuckerberg faced lengthy questioning by Congress. And then a blistering New York Timesexposé accused Facebook's leaders of attempting to keep evidence of Russian meddling in the 2016 election a secret from the public and Facebook's own board more than a year after the company's engineers discovered it.

Under the pressure of all this bad news, Facebook's reputation as the company where everyone wanted to work and no one wanted to leave began showing some cracks. Zoom replaced it at the top of Glassdoor's list of tech workplaces, as Facebook dropped to a still very enviable seventh place. But then former Facebook employees began reporting that they were getting phone calls from their one-time Facebook co-workers, asking for help finding employment elsewhere.

Now, a dozen former employees who all asked to remain anonymous have told CNBC that working for the company was a lot like joining a cult. And that Facebook's happy reputation is built in part on a corporate culture that discourages dissent, holds back employees who aren't big enough boosters of the company, and even punishes some who choose to leave for years after they move on.

Admittedly, a dozen employees out of more than 25,000 may not be a representative sampling. And, as ex-employees, they could be biased against the company. But comments like these on Quora, and the Times' account of COO Sheryl Sandberg ripping into network security chief Alex Stamos for daring to tell the board the truth about Russian interference, sure make it sound like what those employees are saying could be true.

"The pressure for us to act as though everything is fine and that we love working here is so great that it hurts," an employee reportedly told Sandberg during a town hall meeting at the company. "There shouldn't be this pressure to pretend to love something when I don't feel this way."

Another employee reported being publicly criticized because she failed to attend a team-building event outside work hours. At the time, the employee was going through a difficult divorce, but she says her manager faulted her anyway for her non-attendance. (A Facebook representative told CNBC that attendance at after-hours events is not mandatory, while also noting that the company considers collaboration important.)

Then there are the Facebook posts of Facebook employees themselves, which tend to be glowing and to share positive news about the company. Those may or may not reflect how they genuinely feel, former employees explained. "People are very mindful about who they're connected with on Facebook who they also work with, and how what they're posting will put them in a favorable light to their managers," one said.

Facebook employees not only have to please their managers, but also their peers. Each employee must seek out five co-workers to evaluate him or her as part of the review process, and what these co-workers say weighs heavily in their score. That means making friends and forming alliances is nothing short of a survival skill at the company. It creates further pressure to avoid dissent or conflict of any kind for fear of alienating a colleague from whom you might need a positive review.

Stack Ranking: 15 percent of Facebook in danger of getting fired.

Perhaps the scariest part of working at Facebook is something called "stack ranking," a management technique that's common in Silicon Valley and has been used in the past by both Microsoft and Amazon. In Facebook's version, employees are evaluated twice a year and, based on those evaluations, each employee is placed into one of seven categories. The catch is that each category can only have a limited percentage of employees within it. As CNBC explains, the top four categories, "Redefines," "Greatly Exceeds Expectations," "Exceeds," and "Meets All" can be applied to no more than 85 percent of the workforce. The lower categories, "Meets Most," "Meets Some," and "Does Not Meet," mean an employee may be fired, and yet managers are obliged to give one of these designations to about 15 percent of Facebook's employees. (A Facebook representative told CNBC that the percentages are guidelines, not hard and fast rules.) Even if you don't get fired, former employees say, getting one of these lower ratings will limit your career advancement for the rest of your time at the company.

That time may be longer than you want it to be. Former employees who posted on Quora described getting burned out or feeling survivor's guilt as other employees vanished during the culling that the review system requires. That might have some people heading for the exits, especially in today's tight labor market where anyone who's unhappy with their job can easily find another, particularly in Silicon Valley.

But, anonymous former employees report, if you're a Facebook employee and you want to leave, you have to be very, very cautious about how you do it. They say that employees who leave Facebook are categorized as either "regrettable" or "non-regrettable." If you decide to quit, you do not want to wind up with that second designation. Not only will you never be able to work at Facebook again (which you might not want to do anyhow), it can also limit your opportunities with the other tech giants in the Valley.

Thus, former employees who've successfully moved on say they are fielding calls from current Facebook employees asking for advice about how to leave the company without incurring that "non-regrettable" black mark against them. It isn't easy. "The way you do it and the timing matters a lot, and it requires knowledge of the game," one former Facebook engineer told CNBC.

Add it all up. If what these former employees say is true, once you join this group of people you must keep its secrets. You must give it unflinching loyalty and be available both during and outside of work hours. You should not openly disagree with either your bosses or your peers. You must express enthusiasm and love for the organization at all times, both internally and publicly. You are in constant danger of being cast out, and if you are cast out, you may have a hard time finding a home someplace else. If you decide you want to leave, you must do so very carefully or risk having your future well-being affected.

That does sound kind of like a cult, doesn't it?

I reached out to Facebook for a response. Spokesperson Anthony Harrison sent the following comment:

Facebook is consistently ranked among the top ten best places to work on Glassdoor's employee-driven Best Places to Work list and has ranked in the top spot twice. To suggest that the experiences of a handful of disgruntled, anonymous employees speaks for everyone is ludicrous. We encourage open discussion, regular feedback, and honest conversation around what is and isn't working at Facebook. While no performance review program is perfect, we continually evaluate ours to best support the changing needs of our people and the company as a whole​.